Mixing process of groundwater in boreholes is analyzed through laboratory experiment. Initial instability is set up by putting brine water on fresh water in a model borehole. At the first stage, brine water migrates into fresh water with a front moving with a velocity depending on the diameter of the model borehole. The second stage of mixing is characterized by the mixing process which can be well described by the equation of diffusion with constant diffusivity. The Diffusivity takes different values in boreholes with different diameter, but it depends little on the density gradient.